It is well known in the art to use various types of minerals as paper coating and filler pigments. In particular, sodium aluminosilicates are well known as paper fillers, as are various forms of clay such as kaolin. It is also known in the art to modify minerals by application of surface coatings of various inorganic compositions. Thus, U.S. Pat. No. 3,849,149 to Swift et al, 1974, discloses a method of modifying the surface properties of particulate mineral materials by forming on the surface of the particles a coating which has a significant number of acidic sites. According to this patent, the surface of certain natural aluminosilicate minerals including kaolin and bentonite can be modified to give an enhanced acidic reaction by a procedure involving heat treatment. This patent also discloses that the surfaces of such clay mineral particles can be provided with acidic coatings to achieve reproducibly a variety of acidic surfaces. This patent discloses a pigment or filler material consisting of a particulate mineral such as kaolin or bentonite having a surface coating which has a significant number of acidic sites which have PKA values of less than 2.8. A main object of this patent is to place the acidic surface coatings on the mineral so that additional coatings of organic materials such as silanes can be applied to the mineral.
U.S. Pat. Nos. 4,026,721, 4,072,537, and 4,117,19 to Kurrle, disclose a composite silicate pigment for use in paper prepared by a precipitation reaction wherein spherical, hydrous metal silicate particles are precipitated on the planar surfaces of clay particles which have a platelet type structure. This composite pigment in the specific embodiment of these patents is a kaolin clay component having a platelet type structure with a planar surface and having a metal silicate component in the form of spherical particles of an alkaline earth metal silicate precipitated on the planar surfaces of the clay platelets. According to this patent, when incorporated into paper, the metal silicate particles act as spacers between individual clay particles to create additional air interfaces on sheet drying.
In published PCT Application No. WO 87/07884, published December 30, 1987, of this assignee, there are disclosed compositions which may be used as paper pigments which comprise reaction products of kaolin with an alkali metal silicate or similar reactant. In these compositions, there is produced an altered kaolin nucleus which has a rim, edge or border which comprises the reaction product of the kaolin and the alkali metal silicate or similar reactant. Thus this product is not a composite pigment but rather is the reaction product of the kaolin with the alkali metal silicate or similar reactant.
The present invention provides an improved coated mineral pigment which is useful as a paper pigment, coating pigment and filler which is distinct from these prior art compositions.